In truth, they are not useful under normal means. However, they can be useful for some things. If you're running out of MP, you can try using Syphon on the Esper for some free MP. If you want to level up faster, you can summon an Esper so the summoner will gain more exp.

Other than that, I only used Belias once in a battle to totally annihilate (SPOILER)Vossler.